As optical networks increasingly carry burgeoning Internet traffic, the need for advanced and efficient optical components rises. Optical communication systems permit the transmission of large quantities of information. Improved optical integrated circuits (OICs) are particularly needed. OICs come in many forms such as 1×N optical splitters, optical switches, wavelength division multiplexers (WDMs), demultiplexers, optical add/drop multiplexers (OADMs), and the like. Optical circuits allow branching, coupling, switching, separating, multiplexing and demultiplexing of optical signals without intermediate transformation between optical and electrical media.
Such optical circuits include planar lightwave circuits (PLCs) having optical waveguides on flat substrates, which can be used for routing optical signals from one of a number of input optical fibers to any one of a number of output optical fibers or optical circuitry. PLCs make it possible to achieve higher densities, greater production volume and more diverse functions than are available with fiber components through employment of manufacturing techniques typically associated with the semiconductor industry. For instance, PLCs contain optical paths known as waveguides formed on a silicon wafer substrate, wherein the waveguides are made from transmissive media which have a higher refractive index than the chip substrate or the outlying cladding layers in order to guide light along the optical path. PLCs are fashioned to integrate multiple components and functionalities into a single optical chip.
One important application of PLCs specifically and OICs generally involves wavelength-division multiplexing (WDM) including dense wavelength-division multiplexing (DWDM). DWDM allows optical signals of different wavelengths, each carrying separate information, to be transmitted via a single optical channel or fiber in an optical network. In order to provide advanced multiplexing and demultiplexing (e.g., DWDM) and other functions in such networks, arrayed-waveguide gratings (AWGs) have been developed in the form of PLCs.
A problem with OICs/PLCs is polarization dependence of the waveguides, typically caused by thermal stress induced waveguide birefringence. Such birefringence is experienced in varying degrees with waveguide fabrication process. The difference in thermal expansion coefficient between the waveguide top cladding layer and the substrate causes thermal stress. That stress imposed on the waveguide core in a direction parallel to the surface usually is different from that in a perpendicular direction. When the stress is asymmetric to the waveguide core, birefringence is induced undesirably rotating the optical axes.
Stress induced waveguide birefringence results in a difference of refractive index of the waveguide in the direction between parallel and perpendicular to the waveguide. The birefringence, in turn, causes polarization dependence in the waveguides, where the propagation constant for TE (transverse electric) mode is different from TM (transverse magnetic) mode. Consequently, the device characteristics change in accordance with the polarized state of the light provided to the device. For AWG device, this difference in propagation constants results in a wavelength shift in the spectral response peak or the passband of each wavelength channel. A conventional AWG may exhibit a polarization dependent wavelength shift of 0.1 nm, which is sufficient to undesirably impact the performance of a PLC containing the AWG.
One method of reducing thermal stress induced birefringence and resultant polarization dependent wavelength shift involves matching the coefficient of thermal expansion of the top cladding with the coefficient of thermal expansion of the substrate. This can be accomplished by doping the top cladding with boron, if silicon wafer is used as substrate. However, high boron concentrations in the top cladding lead to corrosion problems.
This polarization sensitivity or dependence in AWGs and other dispersive components can be minimized by bisecting the waveguides with a half waveplate, in a slot between waveguide portions. The half waveplate causes polarization swapping partway along the optical paths of the bisected waveguides, such that any input polarization samples each propagation constant equally and provides essentially no shift in peak wavelength with changes in input polarization. Thus, the spectrum for the TE and TM modes coincide through the use of the half waveplate.
However, there are concerns with the use of half waveplates. For instance, although the conventional use of the half waveplate reduces the polarization sensitivity problems associated with waveguide birefringence, back reflection is increased and the mere presence of a half waveplate bisecting the waveguides generates insertion loss. Insertion loss is the total optical power loss caused by the insertion of an optical component, such as a half waveplate in this instance, into an optic system. A half waveplate bisecting the waveguides of an AWG can introduce an insertion loss of 0.5 dB.
Since most conventional AWG's are extremely sensitive to temperature variations, another method of reducing thermal stress induced birefringence and resultant polarization dependent wavelength shift involves using a heater in conjunction with a temperature sensor to control the temperature of the device, particularly in hot or cold ambient environments. The metric used to quantify the sensitivity of an optical circuit is referred to as the center wavelength shift over temperature. For example, a typical silica based AWG without temperature control has a center wavelength sensitivity of 11.0 picometer per degree C.
The current conventional thin film heaters are designed to provide constant flux heat input. However, a constant flux heater cannot heat the surface of the AWG as uniformly as desired. Referring to FIG. 1, a conventional constant flux heater 100 is shown. The heater 100 configuration contains two heating elements 110 and 120 centered on a metal plate 130. FIG. 2 shows a resulting temperature distribution 200 when the conventional constant flux heater 100 is attached to the metal plate 130 at 1.687 watts of heater power in an ambient temperature of −5° C. The temperature isotherms are circular and relatively tight. For example, as depicted in FIG. 2, different portions of the constant flux heater 100 have different temperature zones 1–17. Thus, the constant flux heater 100 does not contain a uniform temperature area of sufficient size for an AWG. Table 1 illustrates the different temperatures zones 1–17 for the constant flux heater at 1.687 watts and −5° C.
TABLE 1ZoneTemperature (° C.)183.922283.864383.807483.750583.692683.635783.577883.520983.4631083.4051183.3481283.2901383.2331483.1751583.1181683.0611783.003Thus, an AWG coupled to the constant flux heater 100 can experience temperature variations ranging from about 0.570° C. to about 0.919° C.
Another method of reducing temperature variations across an AWG involves changing materials that constitute the AWG (cladding, substrate layer, waveguide material, and the like). One disadvantage to using different materials is that the problems associated with using a constant flux heater design are not addressed. Moreover, the materials often used to minimize the temperature variations can be expensive and difficult to obtain.
Yet another method of reducing temperature variations across an AWG involves using seperate, relatively small heaters, and separating the small heaters across the device. One disadvantage to using separate, small heaters is that the problems associated with using a constant flux heater design are not addressed. In addition, using separate, small heaters often involves differences in resistance from heater to heater that can change the local temperature profile.
Polarization dependence of optical network components, such as polarization dependent wavelength shift in AWGs affect a system's performance, especially when there are many components in the system. Consequently, there remains a need for better solutions to reduce temperature sensitivity in general and polarization dependence in particular in OICs/PLCs such as AWGs.